<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>
	<body>
		
		<canvas id="canvas" width="400" height="300" style="border:1px solid silver;"></canvas>
		<button id="button">清除画面</button>
		<script>
			window.onload=function(){
				var canvas=document.getElementById("canvas");
				var cxt=canvas.getContext("2d");
				var canvasx=canvas.getBoundingClientRect().left;
				var canvasy=canvas.getBoundingClientRect().top;
				console.log(canvasx);
				console.log(canvasy);
				canvas.addEventListener("mousedown",downfun);
				canvas.addEventListener("mousemove",movefun);
				canvas.addEventListener("mouseup",upfun);
				var startx;
				var starty;
				var endx;
				var endy;
				var offen=false;
				cxt.lineWidth=3;
				function downfun(e){
					offen=true;
					startx=e.clientX-canvasx;
					starty=e.clientY-canvasy;
					
				}
				
				function movefun(e){
					if(offen=true){
						cxt.beginPath();
						endx=e.clientX-canvasx;
						endy=e.clientY-canvasy;
						cxt.moveTo(startx,starty);
						cxt.lineTo(endx,endy);
						cxt.stroke();
						cxt.closePath();
						startx=endx;
						starty=endy;
						console.log("111")
					}
					
				}
				function upfun(){
					offen=false;
					cxt.closePath();
				}
				document.getElementById("button").onclick=function(){
					cxt.clearRect(0,0,canvas.width,canvas.height);
				}
			}
		</script>
	</body>
</html>
